1. Field of the Invention
The present invention relates to electromagnetic filters and, more particularly, to improved multi-cavity dielectric filters for attenuating signals in the ultra-high frequency range.
2. Description of the Prior Art
Dielectric filters typically are used for filtering electromagnetic energy in the ultra-high frequency band, such as those used for cellular communications in the 800+ MHz frequency range. Band reject filters often comprise a plurality of dielectric notch resonators that are coupled to a transmission line by means of well-known coupling techniques. Bandpass filters also often comprise a plurality of dielectric resonators.
Representative of such filters are the filters shown in U.S. Pat. No. 4,862,122, entitled, "Dielectric Notch Filter", issued Aug. 29, 1989 and U.S. Pat. No. 5,065,119, entitled, "Narrow-Band, Band-Stop Filter", issued Nov. 12, 1991.
These filters are designed and manufactured having a plurality of dielectric resonators with each dielectric resonator having its own housing and each housing having top and bottom covers and cylindrical or rectangular sidewalls. Each housing serves to contain electromagnetic fields thereby preventing radiation losses that would lower the quality factor (Q) of the resonator. The Q is also related to the internal dimensions and the conductivity of each housing. The resonators in the case of notch filters are positioned along a transmission line at intervals of an odd multiple of a quarter wavelength as determined by the center of the filtering frequency. The transmission line serves to couple the resonators thereby producing the desired frequency response. In the bandpass case the resonators are usually proximity coupled, within input and output connectors and associated coupling loops rather than through use of a transmission line and associated coupling loops.
A shortcoming of these filters is that each resonator requires its own individual housing, thereby resulting in a less than optimum filter size and high material costs.
Although U.S. Pat. No. 5,051,714, entitled "Modular Resonant Cavity, Modular Dielectric Notch Resonator and Modular Dielectric Notch Filter", describes a modular dielectric notch filter, the overall housing comprises a plurality of individual shells 24 or 24' that are secured together by means of rods 42. The closure plates 26, 26' and 26" securely mechanically interfit with the ends of the shells. There is no suggestion that the closure plates need not be securely mechanically interfitted to the shells, nor that the shells could be combined into a single housing. Furthermore, the disclosed orientation of resonators 48 would generate current flow in plates 26, thereby requiring a continuous mechanical (and therefore electrical) connection with shell 24.
It would be desirable to overcome the above-mentioned shortcoming of each resonator having its own individual housing. It would also be desirable to have a multi-cavity filter that is easier to fabricate than the multi-housing filter design shown in U.S. Pat. No. 5,051,714. Accordingly, an improved multi-cavity filter having a single housing for a plurality of dielectric resonators is disclosed herein.